This invention relates to a pneumatic tire which has a rubber tread produced from conventional tread elastomers on a cast, fabricless, body of high modulus elastomeric material. Such tires, as well as tires having high modulus elastomeric bodies and treads, are generally known. A great deal of emphasis is being placed on tires of these types in which a high modulus elastomer is molded, either by centrifugal molding, injection or transfer molding, in a tire mole without any fabric reinforcement in the body. This method is simple and low cost. It represents a significant savings in labor and equipment over the prior, laborious methods of building tire bodies which comprises laying several layers of fabric-reinforced sheets on top of one another.
A tire which is made entirely of the high modulus elastomeric material has presented service problems due to its lack of durability, road traction and relatively fast tread wear. Sudden brake application or severe cornering with these tires is particularly troublesome as it generates too much heat which gives high rates of tread wear or tread rub-off. These problems have been lessened by substituting a rubber tread for the high modulus elastomeric tread in the tire. This substitution has not necessarily solved the durability and wear problems and has caused other problems, such as adhesion between the rubber tread and the high modulus elastomeric body and excessive inflation growth due to replacing a relatively high modulus material with a relatively low modulus material.
The tire of this invention solves these problems of durability and growth and, as a result, yields better tread wear. The tire of this invention accomplishes this by a definite shape in which the structural stiffness of critical sections of the body are defined and related to each other so that the tire operates similar to the radial tire principle; that is, supple, relatively flexible sidewalls attached to, but functionally decoupled from, a more rigid belt member which is located in the tire crown under the area that contacts the road surface.
The tire of this invention has a unique cross-sectional shape for the tire body. This provides the proper relationships in the critical sections of the tire. The critical sections are the mid-sidewalls of the body, the shoulders of the body and the crown of the body. The structural stiffness of these sections must bear certain relationships to the structural stiffness of the other sections to obtain the structural geometry of the tire and the advantages of this invention. The location of these sections in relation to each other is also critical.
In the construction which contains a rubber tread, it has been determined that the presence of the rubber tread of conventional tread rubber material is not a factor in determining the ratios of these critical sections. Only the body is considered due to the fact that the rubber tread has an extremely low modulus as compared to the high modulus of the elastomeric material of the body.
In all high modulus elastomeric material tires the tread has a significant effect on the structural geometry of the tire because its modulus would be approximately equal to the modulus of the body material. More importantly, this effect would significantly change during the service life of the tire as the tread as progressively worn away thereby progressively changing the structural geometry of the tire during its life. This change, which cannot be compensated by design, is at least partially responsible for the relatively unsatisfactory commercial progress that the cast tire has heretofore attained. In the tire of this invention the structural geometry of the tire in the crown region and the mid-sidewall region is important and remains relatively unchanged during the service life of the tire since the wearing element, the rubber tread, has relatively little effect on the critical structural geometry of the tire at these locations. The structural geometry of the shoulder is changed to a greater degree, but not as much as in an all high modulus elastomeric material tire. So defined, the tire of this invention is able to retain satisfactory performance characteristics during the service life of the tire. This means the tire retains the essence of the radial tire principle during its service life, not just when it is new. This factor yields a great improvement in tread wear and durability.
It is an object of this invention to obtain a feasible, serviceable pneumatic tire having a cast body of high modulus elastomeric material and a rubber tread of conventional tread rubber material to realize the cost advantages of the cast tire systems.
It is a further object of this invention to provide a cast tire body that will not significantly change in its structural geometry during the service life of the tire.
It is a further object of this invention to provide a tire having a fabricless, cast body and a rubber tread which operates similar to the radial tire principle during its service life.
It is a further object of this invention to provide a cast tire which retains satisfactory performance characteristics during the service life of the tire.
Other objects of this invention will be evident from the entire description of this invention which follows.